What is a Coherent Flow Structure in Geophysical Flow?

Definitions exist for what constitutes a coherent flow structure (CFS) in relatively low Reynolds number, smooth boundary flows, but there is no comprehensive nomenclature for the range of dominantly turbulent motions that can be observed in flows at, and near, the Earth's surface. Here, we discuss what defines a coherent flow structure in a geophysical flow and find that the structures typically referred to as CFS have some common characteristics. They generate Reynolds stress and turbulence intensity, and possess vorticity, which implies a rotational aspect to the structure. However, these are not defining characteristics of a CFS because there are other structures that exhibit some or all of these properties, such as persistent recirculation cells or secondary flow cells in rivers. What does appear important is that they must have temporal coherence and spatial coherence in addition to these properties, but the structures must also not be persistent features of the mean flow. We expand towards a broader definition of CFS beyond the fundamental structures identified in low Reynolds number, smooth boundary, flows to include the larger scale structures in geophysical flows (such as macroturbulent kolks and surface boils) and structures formed by shear instabilities. We also include morphological-scale motions like long-wavelength pulsations described by Marquis and Roy (Chapter 17, this volume) and the superstructures described by Marusic et al. (2010).